Type 2 diabetes is a known risk factor for heart disease, and researchers thought that five years of really tight blood sugar control might reduce the risk of heart disease for years to come.
But a new 15-year follow-up study found that was not the case. The findings suggest it might be more important to control other risk factors for heart disease, such as high blood pressure, cholesterol and unhealthy weight.
"Glucose-lowering has a modest benefit, but it's not enough by itself. We need to address all the other cardiovascular risk factors," said the study's lead author, Dr. Peter Reaven. He's a staff physician at Phoenix Veterans Affairs Health Care System, in Arizona.
Reaven emphasized that these findings apply to this particular population -- people who had type 2 diabetes for more than 10 years on average before the study began. Their average age was around 60, and the study group was nearly all men.
So it's not yet clear what the results might be if a blood sugar-lowering intervention was started soon after diagnosis with type 2 diabetes. It's also not known if these findings apply to people with type 1 diabetes.
The initial phase of the study included almost 1,800 people. They were randomly selected to receive either standard diabetes treatment or intensive blood sugar-lowering treatment.
Both groups received the same medications. The difference was in the dosing. People in the intensive group were given enough medication to lower their hemoglobin A1C levels below 7%. Hemoglobin A1C is a blood test that provides an estimate of average blood sugar levels.
An A1C of under 5.9% is considered normal. Researchers aimed to get the intensive treatment group to below 7%. Reaven said the initial average A1C of the whole group was about 9%.
During the intervention part of the study, the group getting usual care lowered their A1C to 8.4%. The intensive group had an average A1C of 6.9%. The intervention lasted nearly six years.
Approximately 10 years after the study began, the researchers found a 17% drop in the risk of heart disease and stroke.
"We wanted to learn if there was a continued benefit, a 'legacy' effect," Reaven explained. And, at least in the short-term, it appeared there might be.
But the 15-year follow-up found no statistically significant difference in the rate of heart attacks and strokes, or deaths.
"This was a fairly definitive assessment, and in this group of older type 2 diabetes patients, there's no evidence of a legacy effect," Reaven said. "Our data suggests that for glucose-lowering to have continued benefit, it must be sustained."
Since the intervention portion of the study was done, newer medications -- some that have specifically been shown to reduce the risk of heart disease -- have been introduced for type 2 diabetes. It's not clear what the long-term effect of these medications might be, or if these newer drugs have a legacy effect.
Results of the study were published June 6 in the New England Journal of Medicine.
Dr. Kasia Lipska from Yale School of Medicine is co-author of an editorial in the same issue of the journal.
"We've learned over time that how you lower blood glucose matters when it comes to cardiovascular risk. With newer medicines that have different mechanisms of action, the benefits for cardiovascular disease are pretty quick," she said.
But she said that controlling other risk factors for heart disease and stroke is crucial. "Very tight control of glucose for cardiovascular disease is not the best way to reduce risk. Avoiding smoking, blood pressure control, statin therapy [to lower cholesterol] are known to reduce cardiovascular disease risk," Lipska said.
That doesn't mean that controlling blood sugar isn't important, it is. But there needs to be a balance. People shouldn't be trying to lower their blood sugar so much that they end up having a dangerous low blood sugar episode.
"Patients need to have a conversation with their doctor about what is right for them," she advised.
Dr. Gerald Bernstein is program coordinator at Lenox Hill Hospital's Diabetes Institute, in New York City. He pointed out -- as did the study authors -- that widespread use of blood pressure and cholesterol medications may have affected the potential benefits from lowering blood sugar. He said this study's findings shouldn't change current blood sugar management.
"The upshot is, control of blood glucose is better than not," Bernstein said.
Learn more about managing type 2 diabetes from the American Diabetes Association.